The present invention relates to a conductive paste for via conductor which is fired together with a green sheet containing a ceramic component to form via conductors in the state of being filled in through holes formed in the green sheet, and to a ceramic wiring board comprising via conductors formed of the conductive paste for via conductor, and a method of manufacturing the ceramic wiring board.
When manufacturing ceramic wiring boards constituting electronic components such as laminated ceramic capacitors, particularly those having a laminated structure of two or more layers, such a design is increasingly employed in recent years that conductor wirings provided in the laminated structure are conductively connected to via conductors which penetrate through the ceramic wiring board in the lamination direction, in order to decrease equivalent series resistance and equivalent series inductance of the conductor wirings.
FIG. 5 is a cross-sectional view showing an example of a laminated ceramic capacitor 90 having the structure described above.
Referring to FIG. 5, the laminated ceramic capacitor 90 of this example is provided with a dielectric block 91 comprising a plurality of dielectric layers 92 made of ceramics laminated one upon another.
A plurality of internal electrodes 93, 94 are provided as conductor wirings alternately between the respective dielectric layers 92 which constitute the dielectric block 91.
The dielectric block 91 has via conductors 95, 96 formed to penetrate therethrough in the lamination direction, and has an external electrode 97 which is conductively connected to the via conductor 95 and an external electrode 98 which is conductively connected to the via conductor 96, both formed on the surface of the dielectric block 91.
The internal electrode 93 is provided on the upper surface of the dielectric layer 92 which is the even-numbered layer, assuming that the uppermost layer is the first layer, in the dielectric block 91 shown in the figure, and is conductively connected to the via conductor 96 and the external electrode 98 in such a configuration as an area 91a without any electrode formed therein is provided around the via conductor 95 so as to isolate from the via conductor 95.
The internal electrode 94 is provided on the upper surface of the dielectric layer 92 which is the odd-numbered layer (except for the uppermost layer, namely the first dielectric layer 92), assuming that the uppermost layer is the first layer, in the dielectric block 91 shown in the figure, and is conductively connected to the via conductor 95 and the external electrode 97 in such a configuration as an area 91b without any electrode formed therein is provided around the via conductor 96 so as to isolate from the via conductor 96.
Thus a circuit which functions equivalently as a capacitor is constituted in which the internal electrode 93 which is conductively connected to the external electrode 98 through the via conductor 96, and the internal electrode 94 which is conductively connected to the external electrode 97 through the via conductor 95 are disposed to oppose each other via the dielectric layer 92 in the region X shown in the figure.
The dielectric layer 92 is formed by firing a green sheet containing a ceramic component and an organic binder resin.
The internal electrodes 93, 94 are formed by applying a conductive paste for conductor wiring, which contains Ni as a conductive component and an organic binder resin, onto the surface of the green sheet by screen printing, for example, in a predetermined planar configuration and firing the paste together with the green sheet.
The via conductors 95, 96 are formed by firing the conductive paste for via conductor together with the green sheet in the state of being filled in the through holes formed in the green sheet.
As the conductive paste for via conductor, a conductive paste containing a Cu powder, a Ni powder, a ceramic component and an organic binder resin is preferably used as described in Japanese Unexamined Patent Publication No. 2003-229325.
In the conductive paste for via conductor described above, the Cu powder forms a solid solution with the Ni powder when fired, so as to suppress the via conductors 95, 96 from shrinking due to firing, thereby to form satisfactory conductive connection of the via conductors with the internal electrodes 93, 94.
The ceramic component suppresses sintering of the conductive paste for via conductor from proceeding faster than firing of the green sheet during firing.
Therefore, cracking can be prevented from occurring in the laminated ceramic capacitor 90.
In the firing step to form ceramic wiring boards such as the laminated ceramic capacitor 90 described above, introducing oxygen of a predetermined concentration in the course of firing is a common practice for the purpose of fully burning and decomposing the organic binder resin contained in the green sheet and in the conductive paste which may otherwise cause internal defects such as delamination.
Also, the formation of the Cu—Ni alloy from Cu and Ni metal powders during sintering increases the electrical resistance of the via conductor and deteriorates the electrical conductivity.